Do it as often as you want - sometimes I do this within the construct of a TR plan (I sub-out workouts) and other times I’m fine knowing that if my FTP has gone up and I’m actually working at tempo, my aerobic base is building and I’m becoming faster through stamina/repeatability.
There is so much sub-threshold/aerobic work to do for the time-crunched athlete, none of it is a waste — all of it makes you faster, just in different ways.
In my opinion, unless you’re ramping towards an event and in an 8-10 week window outside of it… push more and more time in zone.
My personal best is 2x90 @ <70 RPM. My FTP didn’t go up, but I went to a 5 day riding camp and no one could hang with me except Lael Wilcox and Dylan Morton (look them up) - I just had stamina for days. Felt like I could do whatever I wanted on a bike…
Lael and Dylan were cool to me. They also dropped me when they wanted… but I got to ride with them🤘